AN3302

Test results

Doc ID 18163 Rev 1

9/13

3.2

Inductor selection

The inductor is the key passive component for switching converters. The inductor selection

must take the boundary conditions in which the converter works into consideration; for the

buck, the maximum input voltage, and for the boost, the minimum input voltage. The critical

inductance values are then obtained according to the following formulas:

for the step-down:

Equation 3

and for the step-up:

Equation 4

where:

ΔI

can be set at 20 % - 40 % of the output current for the step-down and can be set at 20 % -

40 % of the input current for the step-up.

The peak current of the inductor must be calculated as:

Equation 5

Equation 6

In addition to the inductance value, in order to avoid saturation, the maximum saturation

3.3

Input and output capacitor selection

It is recommended to use ceramic capacitors with X5R or X7R dielectric and Iow ESR as

input and output capacitors in order to filter any disturbance present in the input line and to

obtain stable operation. The output capacitor is very important to satisfy the output voltage

ripple requirement.

(

)

L

SW

MAX

_

IN

OUT

MAX

_

IN

OUT

MIN

I

F

V

V

V

V

L

Δ

⋅

⋅

−

⋅

=

(

)

L

SW

OUT

MIN

_

IN

OUT

MIN

_

IN

MIN

I

F

V

V

V

V

L

Δ

⋅

⋅

−

⋅

=

()

(

)

L

F

V

2

V

V

V

8

.

0

I

I

SW

MAX

_

IN

OUT

MAX

_

IN

OUT

OUT

DOWN

STEP

PEAK

⋅

⋅

⋅

−

⋅

+

=

−

−

(

)

L

F

V

2

V

V

V

V

I

V

I

SW

OUT

MIN

_

IN

OUT

MIN

_

IN

MIN

_

IN

OUT

OUT

UP

STEP

PEAK

⋅

⋅

⋅

−

⋅

+

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⋅

η

⋅

=

−

−